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Discovery

Deimos was discovered by the astronomer Asaph Hall on August 12, 1877, at the United States Naval Observatory in Washington, DC.[6][7][8] The astronomer V. Knorre named the satellite Deimos (and also provided the name Phobos for the other satellite that Hall discovered six days later), per a suggestion by Henry G. Madan of Eton, based on the names given in The Iliad for the two servants of Ares, the Greek god of war, named Fear (Phobos) and Panic (Deimos).[9]

Orbital and physical characteristics

Deimos orbits Mars at a distance slightly further away than the distance of a synchronous orbit. For that reason, Deimos rises in the east and sets in the west of the Martian sky, about 2.7 days after its rising.[10]

Deimos is not round, but is shaped like a potato, with dimensions 15 x 12.2 x 11 km. Its largest surface feature is a 2.3 km diameter crater.[10] Deimos is heavily cratered but has a smooth-appearing surface. Its surface gravity is very weak, perhaps too weak to retain the ejecta from a crater impact. This ejecta is likely retained around Mars in a ring and redeposited as regolith on the surface of Deimos as it passes.[11]

Deimos' orbit is so little inclined with respect to the ecliptic that it makes daily transits across the Sun.[10]